Single facet laser sources

ABSTRACT

The embodiments herein describe a single-frequency laser source (e.g., a distributed feedback (DFB) laser or distributed Bragg reflector (DBR) laser) that includes a feedback grating or mirror that extends along a waveguide. The grating may be disposed over a portion of the waveguide in an optical gain region in the laser source. Instead of the waveguide or cavity being linear, the laser includes a U-turn region so that two ends of the waveguide terminate at the same facet. That facet is coated with an anti-reflective (AR) coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/953,250, filed Apr. 13, 2018. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments presented in this disclosure generally relate to a laserwith a feedback grating and outputs at a shared facet.

BACKGROUND

Distributed feedback (DFB) lasers typically include a first facet whichhas a partial reflector (i.e., an anti-reflective (AR) coating) and asecond facet which has a total reflector (i.e., a high reflective (HR)coating). While the second facet with the total reflector reflects morethan 90% of the light in the laser, this configuration results in lowthreshold current. Further, the second facet is typically fabricatedusing a mechanical process (e.g., a cleaving process) which is notprecise (i.e., has high tolerances). As a result, the phase of the lightreflected from the total reflector back towards the partial reflector isuncertain. This grating phase change can cause instability in the HR/ARDFB laser that can result in mode hopping where the wrong wavelength isamplified.

However, using two facets that both have partial reflectors (e.g., theAR coating) can result in low efficiency. That is, although DFB lasersthat use AR/AR coatings are inherently less susceptible to grating phasechanges at the facets, they suffer from reduced efficiency wheresignificant amounts of light exit from both facets in contrast to theHR/AR DFB laser where almost all the light exits through the facet withthe AR coating but not the facet with the HR coating.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIG. 1 illustrates a DFB laser source coupled to a photonic chip,according to one embodiment described herein.

FIGS. 2A-2C illustrate a DFB laser source coupled to a photonic chip,according to embodiments described herein.

FIG. 3 illustrates a DFB laser source coupled to a photonic chip,according to one embodiment described herein.

FIGS. 4A and 4B illustrate a DFB laser source coupled to a photonicchip, according to one embodiment described herein.

FIG. 5 illustrates a DFB laser source coupled to a photonic chip,according to embodiments described herein.

FIG. 6 illustrates a DFB laser source coupled to a photonic chip,according to embodiments described herein.

FIG. 7 illustrates a DFB laser source coupled to a photonic chip,according to embodiments described herein.

FIGS. 8A and 8B illustrate a distributed Bragg reflector laser sourcecoupled to a photonic chip, according to embodiments described herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

One embodiment presented in this disclosure is a laser source thatincludes a facet coated with an anti-reflective (AR) coating, anintermediate region, a U-turn region, wherein the intermediate region isbetween the facet and the U-turn region, and a waveguide extendingthrough the facet, the intermediate region, and the U-turn region. Afirst portion of the waveguide in the U-turn region changes directionsuch that the waveguide exits the U-turn region in an opposite directionthan the portion of the waveguide entered the U-turn region and firstand second ends of the waveguide terminate at the facet. The lasersource also includes a grating disposed over a second portion of thewaveguide in at least one of the intermediate region and the U-turnregion where at least one of the intermediate region and the U-turnregion provides optical gain to a signal propagating in the waveguide.

Another embodiment described herein is an optical system that includes alaser source that includes a facet coated with an anti-reflective (AR)coating, an intermediate region, a U-turn region, and a waveguideextending through the facet, the intermediate region, and the U-turnregion. A first portion of the waveguide in the U-turn region changesdirection such that the waveguide exits the U-turn region in an oppositedirection than the portion of the waveguide entered the U-turn regionand the first and second ends of the waveguide terminate at the facet.The laser source also includes a grating disposed over a second portionof the waveguide in at least one of the intermediate region and theU-turn region. The optical system further includes a photonic chip thatincludes a first optical coupler aligned to the laser source to receivea first optical signal emitted from the first end of the waveguide viathe facet and a second optical coupler aligned to the laser source toreceive a second optical signal emitted from the second end of thewaveguide via the facet.

EXAMPLE EMBODIMENTS

The embodiments herein describe a single-frequency laser source (e.g., adistributed feedback (DFB) laser or distributed Bragg reflector (DBR)laser) that includes a feedback grating or mirror that extends along awaveguide. Instead of the waveguide or cavity being linear, the laserincludes a U-turn region so that two ends of the waveguide terminate atthe same facet, which is coated with an AR coating. As such, the laseravoids the grating phase change which can cause instability in an HR/ARlaser source resulting from reflecting light off of the HR coated facet.

The laser sources described herein also avoid the problem of low opticalefficiency of AR/AR lasers that have two facets since the two ends ofthe waveguide terminates on the same facet. The two ends can then beoptically aligned to optical couplers in a photonic chip. As a result,substantially all of the light is transmitted from the laser source tothe photonic chip. For example, the photonic chip may include an opticalcombiner which combines the light received at the two optical couplersinto a single waveguide, or the two optical couplers can be coupled todifferent optical components in the photonic chip (e.g., to independentmodulators).

FIG. 1 illustrates a DFB laser source 100 coupled to a photonic chip150, according to one embodiment described herein. The DFB laser source100 includes a U-turn region 105, an optical gain region 110 (orintermediate region), and a facet 115. The U-turn region 105 defines alocation of a waveguide 120 in which the light generated by the laserreverses direction such that the two ends of the waveguide 120 exit onthe same facet 115. The optical gain region 110 is a region wherecurrent is used to generate and/or amplify the light. The facet 115serves as the output interface of the DFB laser source 100 and is coatedwith an AR coating 125. Although not shown here, the DFB laser source100 includes a feedback grating which extends along at least a portionof the waveguide 120 and serves as a mirror for reflecting a particularwavelength of light traveling in the waveguide 120 as optical feedback.In one embodiment, the grating relies on Bragg scattering to provide theoptical feedback for the laser source 100. The grating can reflect lightback into the waveguide 120 to form a resonator.

The two ends of the waveguide 120 that terminate in the facet arealigned to respective optical couplers 155 in the photonic chip 150.That is, one end of the waveguide 120 aligns with the optical coupler155A while the other end aligns with the optical coupler 155B. Byterminating both ends of the waveguide 120 on the same facet, the endscan be aligned to the same facet of the photonic chip 150. In contrast,in another embodiment, the U-turn region 105 can be replaced with asecond facet disposed opposite the first facet 115 such that the one endof the waveguide 120 terminates on the first facet 115 and a second endof the waveguide 120 terminates at an AR coating on the second facet. Inthis example, the waveguide 120 is linear and does not change directionas it extends between the two facets. However, the photonic chip may bealigned to both facets in order to capture most of the light generatedby the laser source. Adding the U-turn region 105 in the embodimentsherein avoids having to align the photonic chip to two facets when usingAR coatings and when high optical efficiency is desired.

The optical couplers 155 can include any type of structure whichoptically couples optical components 160 to the DFB laser source 100.For example, the optical couplers 155 may covert a mode size of thelight exiting from the facet 115 into a size that is compatible withwaveguides in the photonic chip 150 that couple the optical couplers 155to the optical components 160. These waveguides may be sub-micronwaveguides. For example, the optical couplers 155 may include one ormore prongs which taper in order to reduce the mode size to better matchthe dimensions of the waveguides in the photonic chip 150. Although FIG.1 illustrates that the ends of the waveguide 120 bend in the facet 115as the ends approach the AR coating 125 which may reduce parasiticreflections, this is not a requirement and the waveguide 120 may notbend or change direction when extending through the facet 115.

The optical components 160 can use the light received from the opticalcouplers 155 to perform any number of different functions such ascombining the light, modulating the light according to a digital datasignal, amplifying the light, transmitting the light to an externaldevice (e.g., an optical cable), and the like. Thus, the opticalcomponents 160 can include optical combiners, modulators, transmitters,etc.

In one embodiment, the photonic chip 150 is formed using a siliconsemiconductor substrate, while the DFB laser source 100 is formed on adifferent semiconductor substrate. For example, the DFB laser source 100may be formed using a combinational semiconductor such as galliumarsenide, indium phosphide, or other III-V semiconductors. Moreover,although the photonic chip 150 is described herein as have a siliconsubstrate, it is not limited to such and can include other types ofsemiconductor material or dielectric material.

As shown, the waveguide 120 has two ends that terminate within the facet115 at the AR coating 125. The AR coating 125 can be selected based onthe light generated by the DFB laser source, which oscillates near theBragg wavelength of the feedback grating. When compared to other typesof lasers, the DFB laser source 100 has a very narrow line width and isstable over a wide range of operating conditions. Moreover, the designof the DFB laser source 100 can have a predefined phase shift (λ/4, λ/8,etc.) to improve mode suppression and avoid too much gain beinglocalized in a particular area of the optical gain region 110.

FIGS. 2A-2C illustrate a DFB laser source 200 coupled to a photonic chip150, according to embodiments described herein. Like the laser source100 in FIG. 1, the DFB laser source 200 includes the U-turn region 105for reversing the direction of a waveguide 210, the optical gain region110 for providing optical gain of the light in the waveguide 210, and afacet 115 for interfacing the two ends of the waveguide 210 withrespective optical couplers 155A and 155B in the photonic chip 150.

The DFB laser source 200 includes a DFB grating 205 (also referred to asa feedback grating) which is disposed over the waveguide 210. That is,FIG. 2A illustrates a top view of the DFB laser source 200 where atleast a portion of the waveguide 210 is occluded or covered by thegrating 205. In this example, the DFB grating 205 extends over theportion of the waveguide 210 in the U-turn region 105 and the opticalgain region 110 but not over the portion of the waveguide 210 in thefacet 115. The DFB grating 205 can have the same width as the waveguide210, but in other embodiments may be slight thinner or wider than thewaveguide 210. Further, the DFB grating 205 forms one continuousgrating. In other embodiments described below, a laser source caninclude two or more discrete gratings disposed over the waveguide 210.

The U-turn region 105 can be a passive or active region. A passiveU-turn region 105 means that optical gain is not added when the lighttravels through the portion of the waveguide 210 in the U-turn region105. However, an active U-turn region 105 means that current is appliedin this region to generate optical gain. In this scenario, both theU-turn region 105 and the optical gain region 110 can amplify the lightin the waveguide 210. In contrast, the facet 115 is a passive region. Inone embodiment, the material of the waveguide 210 in a passive region isdifferent than the material of the waveguide 210 in an active region(whether the U-turn region or the optical gain region 110). For example,the material of the waveguide 210 in the active regions may have adifferent bandgap than the material of the waveguide 210 in the passiveregions. In one embodiment, the material of the waveguide 210 in thepassive and active regions may be different alloys of the underlyingsemiconductor substrate.

In the U-turn region 105, the direction in which the waveguide 210extends changes until the waveguide exits the U-turn region in thereverse direction it did when entering the region 105. Put differently,the waveguide 210 gradually bends in the U-turn region 105 to form aU-shape such that the ends of the waveguide 210 can terminate on thesame facet 115. The radius of the bend in the U-turn region 105 canvary. For example, if the waveguide is formed from a rib or ridge (asshown in FIG. 2C) with a deeper etch (e.g., the rib has a 1-2 micronsheight), the radius of the bend can be shorter, e.g., around 10 microns.However, if the rib is formed with a shallower etch (e.g., the rib isless than a micron tall), the bend radius may be larger, e.g., between10-500 microns. In general, the radius of the bend may depend on theability of the waveguide 210 to sufficiently contain light as thedirection in which the waveguide 210 extends changes.

FIG. 2B illustrates a cross section of the DFB laser source 200 in FIG.2A as defined by the dotted line A-A which extends along one half of thewaveguide 210. As shown, the DFB grating 205 forms a saw tooth patternover the waveguide 210. The characteristics of the saw tooth patterndetermine the wavelength of light that is reflected by the DFB grating205. For example, the pitch of the DFB grating 205 can affect or set theline width or frequency of the light emitted by the laser source 200. Inaddition to the pitch, the gain spectrum of the DFB laser source 200 canalso set the frequency of the light emitted by the laser source 200.

Although a saw tooth pattern is illustrated, the embodiments herein arenot limited to such. For example, the grating 205 may include a squarepattern, a series of separate ridges or lines, a series of slanted cutsor air gaps, and the like. In general, the gratings described herein canhave any shape or be formed from any material which generates a narrowline width or a single longitudinal lasing mode. In addition, thegrating region can be above the gain region, below the gain region, orto either or both sides. When the grating is on the sides, in essencethe grating is periodically making the waveguide wider or narrower alongthe length.

As shown in FIG. 2B, the DFB grating 205 is a continuous grating thatextends over the waveguide 210 in the optical gain region and the U-turnregion but does not extend over the facet. As mentioned above, the DFBgrating 205 serves as a type of mirror for reflecting a particularwavelength of light traveling in the waveguide 120 as optical feedback.

FIG. 2C illustrates a cross section of the DFB laser source 200 in FIG.2A as defined by the dotted line B-B. Specifically, the cross section atdotted line B-B passes through an active region of the DFB laser source200 (e.g., the optical gain region 110) where optical gain is added tothe light. FIG. 2C illustrates an upper cladding 230, the DFB grating205, an upper waveguide 235, an active region 220 (which includesquantum wells or quantum dots), a lower waveguide 240, and a lowercladding 245 disposed on a semiconductor substrate 215 which may be acombinational semiconductor such as an III-V semiconductor. In oneembodiment, the material of the upper waveguide 235 and the lowerwaveguides 240 may be an alloy of the semiconductor substrate 215 suchas an alloy of gallium arsenide or indium phosphide. Moreover, thecladding surrounding the waveguides may be any material with an index ofrefraction different from the index of refraction of the material of thewaveguides such that light remains substantially contained in thewaveguides as it propagates in the waveguides (e.g., in a directioninto, and out of, the page). Moreover, the upper cladding 230 is shapedas a ridge.

The optical gain is provided by a current 225 that flows through the DFBlaser source 200. That is, the active region 220 can include quantumwells or quantum dots (or any other optical active region) which, inresponse to the current 225, generate optical gain. In one embodiment,the current 225 is primarily a direct current (DC) although it may havesome dither or alternating current (AC) component. Although not shown,the DFB laser source may include electrodes above and below thestructure in FIG. 2C to generate the current 225. Unlike in FIG. 2C, across section of the waveguide 210 in a passive region (e.g., the facet115 or a passive U-turn region 105) may not include the quantum wells orelectrodes for generating the current 225. Moreover, as mentioned above,the material of the waveguide may be different in a passive region thanin an active region. Further, a cross section of the waveguide in thedirection established by the dotted line B-B in the facet 115 wouldexclude the DFB grating 205.

FIG. 3 illustrates a DFB laser source 300 coupled to a photonic chip150, according to one embodiment described herein. FIG. 3 illustrates atop view of the DFB laser source 300. Like in FIG. 2A, the DFB lasersource 300 includes a continuous DFB grating 205 disposed over theportions of the waveguide 210 within the U-turn region 105 and theoptical gain region 110 but not in the facet 115. As such, thecross-sections illustrated in FIGS. 2B and 2C would similarly applyhere. However, instead of using a gradual bend in the U-turn region 105to reverse directions of the waveguide 210, the U-turn region 105 caninclude two turning mirrors 305 disposed at two corners.

In one embodiment, the turning mirrors 305 are etched features in thesemiconductor substrate. The turning mirrors 305 may use total internalreflection in order to redirect the light at a ninety degree angle. Byusing two turning mirrors 305, the U-turn region 105 can redirect thelight 180 degrees so that the waveguide 210 exits the U-turn region 105in the opposite direction it entered the region 105. Moreover, theU-turn region 105 can be a passive or active region.

FIGS. 4A and 4B illustrate a DFB laser source 400 coupled to a photonicchip 150, according to one embodiment described herein. FIG. 4Aillustrates a top view of the DFB laser source 400. As above, the DFBlaser source 400 includes the U-turn region 105 (which can be active orpassive), the optical gain region 110, and the facet 115. While the DFBlaser source 400 illustrates using a gradual bend in the waveguide 210within the U-turn region 105, in another embodiment, the region 105 caninclude the turning mirrors 305 to redirect the light in the waveguideas shown in FIG. 3.

In FIG. 4A, a DFB grating 405 is disposed over the waveguide 210 solelywithin the optical gain region 110. That is, the grating 405 does notextend into the facet 115 and the U-turn region 105. Furthermore, thelower portion of the waveguide 210 in the optical gain region 110includes the DFB grating 405 while no DFB grating 405 covers the upperportion of the waveguide 210 in the optical gain region 110. In oneembodiment, it may be difficult to form the DFB grating in the U-turnregion where the waveguide bends or turns, and thus, and embodimentshown here where the DFB grating 405 is disposed in a region where thewaveguide 210 is straight may improve the performance of the grating405. Moreover, a sub-portion of the optical gain region 110 may generateoptical gain in the waveguide 210. For example, the portion of thewaveguide 210 in the lower half of the optical gain region 110 maygenerate optical gain while the portion of the waveguide 210 in theupper half of the region 110 (which does not have the DFB grating 405)does not provide optical gain.

Although the DFB grating 405 in FIG. 4A is smaller than the gratings inthe previous embodiments, the grating 405 is nonetheless sufficient toprovide a narrow line width. Put differently, the DFB laser source 400can have a single longitudinal lasing mode and stable operation with theDFB grating 405. Thus, as the light passes through portion of thewaveguide 210 on which the grating 405 is disposed, the grating 405reflects a narrow band of wavelengths to produce the narrow line width.

The upper portion of the waveguide 210 in the optical gain region 110(which is not covered by the grating 405) may have more optical gainthan the lower portion of the waveguide 210 which is covered by thegrating 405. Thus, the light generated by the DFB laser source 400 mayhave a greater intensity than light generated by the DFB laser source200 in FIG. 2A where the DFB grating 205 is formed over the waveguide210 in the U-turn and optical gain regions 105, 110. However, having ashorter or smaller DFB grating 405 as shown in FIG. 4A may raise thenoise floor in the optical signal generated by the DFB laser source 400relative to the noise floor of the optical signal generated by the DFBlaser source 200.

FIG. 4B illustrates a cross section of the DFB laser source 400 in FIG.4A as defined by the dotted line C-C which extends along one half of thewaveguide 210. While the grating 405 is disposed over the portion of thewaveguide 210 in the optical gain region 110, the portions of thewaveguide 210 along the dotted line A-A in the U-turn region 105 and thefacet 115 are not covered by the grating. In one example, the grating405 in the optical gain region 110 shown in FIG. 4B is the only feedbackgrating in the DFB laser source 400.

As shown by viewing the side of the DFB laser source 400, the DFBgrating 405 has a similar saw tooth pattern as the gratings in previousembodiments. The characteristics of the saw tooth pattern determine thewavelength of light that is reflected by the DFB grating 405. Forexample, the pitch of the DFB grating 405 can affect or set the linewidth or frequency of the light emitted by the laser source 400.

Although a saw tooth pattern is illustrated, the embodiments herein arenot limited to such. For example, the grating 405 may include a squarepattern, a series of separate ridges or lines, a series of slanted cutsor air gaps, and the like. In general, the gratings described herein canhave any shape or be formed from any material which generates a narrowline width or a single longitudinal lasing mode. In addition, thegrating region can be above the gain region, below the gain region, orto either or both sides. When the grating is on the sides, in essencethe grating is periodically making the waveguide wider or narrower alongthe length.

In another embodiment, rather than the DFB grating being disposed solelyin the optical gain region 110, the DFB grating can be disposed solelyin the U-turn region 105. That is, the DFB grating may be disposedwithin the U-turn region 105 and not in the optical gain region 110 orthe facet 115. In this example, the U-turn region 105 is active. Inturn, the optical gain region 110 may be active or passive.

FIG. 5 illustrates a DFB laser source 500 coupled to a photonic chip505, according to embodiments described herein. The DFB laser source 500is shown generically and could be any of the DFB laser sources 200, 300,or 400 illustrated in FIGS. 2A, 3, and 4A. In FIG. 5, the opticalsignals emitted by the DFB laser source 500 are received by the twooptical couplers 155. In turn, these couplers 155 are coupled towaveguides 520 which transmit the optical signals to an optical combiner510. The optical combiner may be a multimode interference (MMI) opticalsplitter/combiner or a Y-branch splitter/combiner. The lengths of thewaveguides 540 may be tightly controlled so that the phases of therespective optical signals align at the optical combiner 510.

The optical combiner 510 transmits the combined optical signal to anoptical component 515 (e.g., a modulator, transmitter, amplifier, etc.).In this manner, the photonic chip 505 receives two optical signals fromthe DFB laser source 500 using the two ends of the waveguides and theoptical couplers 155 and then combines those optical signals into asingle signal before providing the signal to the optical component 515.As a result, the optical system illustrated in FIG. 5 is similar toHR/AR DFB laser source that is coupled to a photonic chip using the ARcoated facet of the laser source. Stated differently, because thephotonic chip 505 combines the two optical signals emitted by the ARcoated facet of the DFB laser source 500 into a single signal, theoptical system has low optical loss like an HR/AR laser but with theadvantage of using a single AR coated facet, which avoids instabilitythat can occur from a mechanical process used to fabricate the HR coatedfacet of an HR/AR laser source. In this manner, the two optical signalsemitted by the DFB laser source 500 can be combined into a singleoptical signal that can be transmitted to the optical component 515.

FIG. 6 illustrates a DFB laser source 600 coupled to a photonic chip605, according to embodiments described herein. The DFB laser source 600is shown generically and could be any of the DFB laser sources 200, 300,or 400 illustrated in FIGS. 2A, 3, and 4A. In FIG. 6, the opticalsignals emitted by the DFB laser source 600 are received by the twooptical couplers 155. In turn, these couplers 155 are coupled towaveguides 520. However, unlike in FIG. 5, here the waveguides 520 arenot attached to an optical combiner but instead form two independentoptical paths. As shown, the optical signal in the waveguide 520A istransmitted to a different optical component 610 than the optical signalin the waveguide 520B which is transmitted to the optical component 615.

For example, the optical components 610 and 615 could be two separatemodulators which modulate respective optical signals received from theDFB laser source 600 using independent electrical data signals. In thisexample, the DFB laser source 600 can be thought of as two differentlaser sources which output two optical signals that can be used inseparate optical paths within the photonic chip 605. The optical signalsgenerated by the DFB laser source 600 may have substantially the samefrequency content (e.g., line width) and intensity. So long as theintensity of the optical signals is sufficient, a single DFB lasersource 600 can perform the same function as two different laser sourcesthat are coupled to the photonic chip 605.

FIG. 7 illustrates a DFB laser source 700 coupled to a photonic chip705, according to embodiments described herein. Unlike in the previousembodiments, the DFB laser source 700 includes an optical combiner 710in the facet 115. That is, instead of the two ends of the waveguideterminating at different locations in the facet 115, the ends of thewaveguides meet (or terminate) at the optical combiner 710 (e.g., a MMIsplitter/combiner or a Y-branch splitter/combiner). The output of theoptical combiner 710 then terminates at the AR coating of the facet 115.

The photonic chip 705 includes a single optical coupler 155 forreceiving the combined optical signal emitted from the optical coupler710 and the facet 115. That is, unlike in the previous embodiments, thephotonic chip 705 can use one optical coupler 155 rather than several.The output of the optical coupler is then coupled to an opticalcomponent 715 (e.g., a modulator, transmitter, etc.). Thus, in FIG. 7,the optical combiner 710 is moved onto the DFB laser source 700 ratherthan in FIG. 5 where the optical combiner 510 is in the photonic chip505. One advantage of doing so is that aligning the DFB laser source 700to the photonic chip 705 may be less complicated since there is onecoupler 155 that is aligned to the DFB laser source 700 rather than two.For example, when using one optical coupler 155, the rotation of the DFBlaser source 700 and the photonic chip 705 along an axis that extendsthrough the output of the optical combiner 710 and the optical coupler155 is less important to ensuring sufficient optical coupling betweenthe laser source 700 and the photonic chip 705.

However, when placing the optical combiner 710 on the DFB laser source700 the phase of the optical signal may be tightly controlled to preventdestructive interference. Further, when the optical signals are combinedin the laser source 700, it prevents the laser source 700 from beingused as two separate laser sources as shown in FIG. 6 unless, forexample, an optical splitter is disposed in the optical chip 705 betweenthe optical coupler 155 and the optical component 715.

In the DFB laser source 700, the U-turn region and the optical gainregion to the left of the facet 115 can be the same as the U-turn andthe optical gain regions discussed above in the DFB laser sources 200,300, or 400 illustrated in FIGS. 2A, 3, and 4A. That is, the U-turnregion in the laser source 700 could have a gradual bend as shown or useturning mirrors. Also, the U-turn region could be active or passive.Further, the DFB grating could be disposed over all of the waveguide inthe U-turn and optical gain regions or over a portion of these regions.

FIGS. 8A and 8B illustrate a distributed Bragg reflector (DBR) lasersource 800 coupled to a photonic chip 150, according to embodimentsdescribed herein. Unlike in the DFB laser sources above, the DBR lasersource 800 includes multiple discontinuous or separate gratings over awaveguide 815 rather than a single, continuous grating. In this example,the laser source 800 includes a DBR grating 810 disposed over a lowerportion of the waveguide 815 in a grating region 820 (or an intermediateregion) and a DBR grating 805 disposed over an upper portion of thewaveguide 815 in the grating region 820. There is not a grating in theU-turn region 105 or the facet 115. As such, the DBR gratings 805 and810 are separate gratings.

In one embodiment, the characteristics and dimensions of the DBRgratings 805, 810 are set such that the DBR laser functions similar tothe DFB laser sources described above where substantially equivalentoptical signals are emitted from the two ends of the waveguide 815.However, in another embodiment, the characteristics or dimension of theDBR gratings 805, 810 are purposefully different so that the opticalsignals emitted from the two ends of the waveguide 815 are different ina desired way—e.g., different wavelengths. For example, by changing thelocal index on one of the gratings, a designer can change the spectralreflectivity of the grating relative to the other grating.

In one embodiment, the grating region 820 (which includes the DBRgratings 805, 810) is passive while the U-turn region 105 is active toprovide the optical gain.

The ends of the waveguide 815 are aligned with respective opticalcouplers 155 in the photonic chip 150. In one embodiment, the outputs ofthe optical couplers 155 are coupled to an optical combiner to generatea combined optical signal which is then transmitted to another opticalcomponent in the chip 150 (like the photonic chip 505 in FIG. 5).Alternatively, the outputs of the optical couplers 155 can be coupled todifferent optical components to form independent optical paths in thechip 150 (like the photonic chip 605 in FIG. 6).

FIG. 8B illustrates a cross section of the DBR laser source 800 in FIG.8A as defined by the dotted line D-D which extends along one half of thewaveguide 815. From this side view, the DBR grating 810 above theportion of the waveguide 815 in the optical gain region 110 is visible.Again, the grating 810 does not extend into the U-turn region 105 or thefacet 115. The shape of the DBR grating 810 is a saw tooth pattern butis not limited to such. For example, the gratings 805 and 810 mayinclude a square pattern, a series of separate ridges or lines, a seriesof slanted cuts or air gaps, and the like. In general, the gratingsdescribed herein can have any shape or be formed from any material whichgenerates a narrow line width or a single longitudinal lasing mode. Inaddition, the grating region can be above the gain region, below thegain region, or to either or both sides. When the grating is on thesides, in essence the grating is periodically making the waveguide wideror narrower along the length.

In another embodiment, the structure shown in FIG. 8A is used to formtwo DFB laser sources (e.g., coupled-cavity lasers) instead of two DBRlaser sources. That is, the grating 805 can be used to generate a firstDFB laser source while the grating 810 generates a second DFB lasersource. Further, the grating region 820 is an active region unlike inthe DBR laser source 800 where the grating region 820 is passive.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. A laser source, comprising: a facet coated with ananti-reflective (AR) coating; an intermediate region; a U-turn region,wherein the intermediate region is between the facet and the U-turnregion; a waveguide extending through the facet, the intermediateregion, and the U-turn region, wherein a first portion of the waveguidein the U-turn region changes direction such that the waveguide exits theU-turn region in an opposite direction than the first portion of thewaveguide entered the U-turn region, wherein first and second ends ofthe waveguide terminate at the facet; and a grating disposed over asecond portion of the waveguide in at least one of the intermediateregion and the U-turn region, wherein the grating is disposed in anoptical gain region that provides optical gain to a signal propagatingin the waveguide, wherein the grating is disposed over a center of thewaveguide in a direction of propagation of the signal in the waveguide.2. The laser source of claim 1, wherein the grating is disposed over athird portion of the waveguide extending through the intermediate regionthat is spaced apart from the second portion of the waveguide extendingthrough the intermediate region, wherein the grating is continuous overthe second and third portions of the waveguide.
 3. The laser source ofclaim 1, wherein the grating is disposed only in the intermediateregion, wherein a third portion of the waveguide disposed in theintermediate region is spaced apart from the second portion of thewaveguide in the intermediate region, wherein the third portion of thewaveguide does not include any grating.
 4. The laser source of claim 1,wherein the laser source includes one continuous grating associated withthe waveguide.
 5. The laser source of claim 1, wherein a material of thewaveguide in the facet is different from a material of the waveguide inthe intermediate region.
 6. The laser source of claim 1, wherein theU-turn region is a passive region.
 7. The laser source of claim 1,wherein the first and second ends are arranged in the facet to emitrespective optical signals to an external optical component.
 8. Thelaser source of claim 1, further comprising: an optical combinerdisposed in the facet, wherein the first and second ends terminate atthe optical combiner, wherein an output of the optical combiner isconfigured to emit an optical signal to an external optical component.9. The laser source of claim 1, further comprising: a semiconductorsubstrate on which the U-turn region, the intermediate region, thefacet, and the waveguide are disposed.
 10. A laser source, comprising: afacet coated with an anti-reflective (AR) coating; an intermediateregion; a U-turn region, wherein the intermediate region is between thefacet and the U-turn region; a waveguide extending through the facet,the intermediate region, and the U-turn region, wherein a first portionof the waveguide in the U-turn region changes direction such that thewaveguide exits the U-turn region in an opposite direction than thefirst portion of the waveguide entered the U-turn region, wherein firstand second ends of the waveguide terminate at the facet; and a gratingdisposed over a second portion of the waveguide in at least one of theintermediate region and the U-turn region, wherein the U-turn regionprovides optical gain to a signal propagating in the waveguide.
 11. Anoptical system, comprising: a laser source comprising: a facet coatedwith an anti-reflective (AR) coating, an intermediate region, a U-turnregion, a waveguide extending through the facet, the intermediateregion, and the U-turn region, wherein a first portion of the waveguidein the U-turn region changes direction such that the waveguide exits theU-turn region in an opposite direction than the first portion of thewaveguide entered the U-turn region, wherein first and second ends ofthe waveguide terminate at the facet, and a grating disposed over asecond portion of the waveguide in at least one of the intermediateregion and the U-turn region, wherein the grating is disposed in anoptical gain region that provides optical gain to a signal propagatingin the waveguide, wherein the grating is disposed over a center of thewaveguide in a direction of propagation of the signal in the waveguide;and a photonic chip comprising: a first optical coupler aligned to thelaser source to receive a first optical signal emitted from the firstend of the waveguide via the facet, and a second optical coupler alignedto the laser source to receive a second optical signal emitted from thesecond end of the waveguide via the facet.
 12. The optical system ofclaim 11, wherein the U-turn region provides optical gain to a signalpropagating in the waveguide.
 13. The optical system of claim 11,wherein the photonic chip comprises: an optical combiner coupled to anoutput of the first optical coupler and an output of the second opticalcoupler and configured to combine the first and second optical signalsinto a combined optical signal; and an optical component coupled to anoutput of the optical combiner to receive the combined optical signal.14. The optical system of claim 11, wherein the photonic chip comprises:a first optical component coupled to an output of the first opticalcoupler; and a second optical component coupled to an output of thesecond optical coupler, wherein the first optical component and firstoptical coupler establish a first optical path that is independent of asecond optical path formed by the second optical component and thesecond optical coupler.
 15. The optical system of claim 11, wherein thesecond portion of the waveguide and the grating extends through both theintermediate region and the U-turn region.
 16. The optical system ofclaim 11, wherein the grating is disposed over a third portion of thewaveguide extending through the intermediate region that is spaced apartfrom the second portion of the waveguide extending through theintermediate region, wherein the grating is continuous over the secondand third portions of the waveguide.
 17. The optical system of claim 11,wherein the grating is disposed only in the intermediate region, whereina third portion of the waveguide disposed in the intermediate region isspaced apart from the second portion of the waveguide in theintermediate region, wherein the third portion of the waveguide does notinclude any grating.
 18. The optical system of claim 11, wherein thelaser source includes one continuous grating associated with thewaveguide.
 19. The optical system of claim 11, wherein a material of thewaveguide in the facet is different from a material of the waveguide inthe optical gain region.
 20. The optical system of claim 11, wherein theU-turn region is a passive region.